This invention relates generally to an optical system generally employed in an optical memory device for use in the field of optical storage and more particularly to an optical system that is sectionalized into a stationary optical system and a movable optical system for tracking and accessing an optical storage medium.
Optical systems for original optical memory devices of the prior art employed an integrated optical head containing an optical system, focus actuator and track actuator all on a single chassis and the movement of these components relative to the chassis for the purpose of providing memory storage tracking and data access. However, the access time was large because of inertia due to the mass of the components that had to be moved. In order to solve this problem, an optical system was provided which was sectionalized wherein the optical storage medium objective focusing lens, the reflecting mirror for the objective lens and focus actuator for the objective lens were provided in a movable optical portion for the optical storage medium access. The remaining components of the optical system, including a galvanomirror for tracking, where provided in a stationary optical portion so that the overall mass and resulting inertia relative to the movable optical portion were significantly reduced.
FIG. 9 shows a side elevation of an optical system of this latter type known in the prior art. Light emitted from semiconductor laser 201 is collimated by objective lens 202 and passes through return beam detection prisms 203 and 204 after which the light path is redirected by reflecting mirror 205 to be incident on the reflecting mirror of galvanomirror 206. All of these foregoing components are part of the stationary optical portion. The light beam is then directed to the movable optical portion comprising reflecting mirror 207 wherein the light beam is redirected to pass through objective lens 105 for focus of the light beam onto a surface of optical storage medium 106. It is important to note that the axis of rotation of galvanomirror 206 is at right angles to the optical axis of the incident light beam on the mirror surface of galvanomirror 206.
Also, in the optical system illustrated in FIG. 9, the light path is divided into two vertical steps or levels in the Y direction relative to the plane of optical recording medium 105 so that the overall size of the optical memory device cannot be made thinner to provide for a more compact size.
Of particular concern is that the optical system of FIG. 9 has an inherent problem of shifting or displacement of the light beam incident on objective lens 105 upon rotation of galvanomirror 206. The adverse effects encountered due to this beam displacement are as follows:
There is a variation in the amount of light incident on objective lens 105 focusing the beam to optical recording medium 106 resulting in a degradation of the accuracy of the energy density on optical storage medium 106, which, in turn, results in write beam failure.
The variance in distribution of light incident on objective lens 105 also results in an unstable focused spot shape resulting in read beam failure.
The diameter of the collimated light beam must made large to prevent shading by objective lens 105. However, this lowers light utilization efficiency and makes it necessary to employ a higher power semiconductor laser 201.
As evident from the cross sectional view of FIG. 10, the position of the return light beam from medium 106 moves with respect to emitted light beam 1501 from a position 1502 relative to its initial position when original incident on galvanomirror 206 to a position 1503 relative to the return beam incident on galvanomirror 206 resulting in the return light beam not being centered on the system servo signal detection sensor resulting in an inaccurate servo signal. This displacement becomes excessive when a track error detection method utilizing the so called push-pull technique is employed. This is because the direction of displacement 1504 of the light beam and partition line 1505 of a two-part sensor employed in the push-pull technique intersect at right angles.
Furthermore, in connection with these prior art systems requiring three reflecting mirrors, the phase difference in the polarized light must be controlled relative to all three reflecting surfaces, particularly in the case of magneto-optical type of recording. Correction of phase differences requires precise control of the formation of multilayer dielectric films deposited on the mirror surfaces. Even in the case where these multilayer dielectric films are carefully formed and their parameters are controlled, the tolerance in the phase difference in the polarized light between reflecting mirrors is realistically about .+-.5 degrees. Therefore, considering a worst case situation comprising a phase difference of .+-.15 degrees relative to the employment of three reflecting mirrors thereby requiring phase control relative to detection prism 204 and an allowable overall polarizing phase difference of approximately .+-.10 degrees, mass production and high yields of such optical systems is extremely difficult to achieve.
FIG. 11 shows further example of a prior art optical system for an optical memory device contained in a standardized size housing 1709. In the case here, the access direction 1708 of movable optical system 1702 is parallel to the plane containing the optical axis of incident light 1706 emitted from semiconductor laser 1701 in stationary optical system 1703. Beam splitter 1705 is positioned in the optical path between light source 1701 and movable optical system 1702 to direct return light beam 1707 from movable optical system 1702 into signal detection optical system 1704, which also comprises a portion of stationary optical system 1703. The overall size of the optical memory device employing standard disc size diameters of 130 mm and 90 mm renders the structure too large, particularly in the horizontal plane of FIG. 11A wherein the rear optics over extend the preferred standardized housing boundary. Furthermore, attempting to fit this structure in a half-height size housing reduces the space available for operation of linear access motors 1710 and 1711 which results in longer access times.
It is an object of this invention to solve the foregoing problems.
It is another object of this invention to provide an optical system for an optical memory device capable of being employed in conventional size memory devices with smaller heights while providing high access speed capabilities.
It is a further object of this invention to effectively eliminate the necessity of controlling the phase differences between polarized light reflected from a plurality of reflecting mirrors employed in an optical system.
It is a further object of this invention to provide an optical system for an optical memory device wherein the amount of displacement between the incident light beam and the return light beam in an optical system brought about by the rotation of a galvanomirror employed in the system is reduced.
It is still a further object of this invention to provide an optical memory device that can be made compact.